1. Field of the Invention
The invention relates to an apparatus for determining the level of a material in a storage silo. More specifically, the invention relates to such an apparatus which includes an AC signal generator whose output frequency is automatically and continuously swept through a predetermined frequency range.
2. Description of Prior Art
Existing devices for automatically measuring the level of material, for example, bulk powdered or granular materials, in storage silos, include capacitance probes, ultrasonic echo detectors, pressure transducers and mechanical range finders. An example of such a device is illustrated in U.S. Pat. No. 4,043,199, Greer, issued Aug. 23, 1977, which is a mechanical device and which detects pressure on a collapsible sleeve 14, which is disposed in a storage silo storing granular material, when the granular material surrounds portions of the collapsible sleeve. U.S. Pat. No. 4,495,807, Field et al, issued Jan. 29, 1985, teaches an electronic device for sensing liquid levels. Specifically, it utilizes a balanced R.F. bridge.
Capacitance probes utilize a conducting rod or cable vertically suspended inside the silo and immersed in the stored material. The electrical capacitance of the probe varies, depending on the level and dielectric properties of the stored material. This variation alters the AC or DC characteristics of an electrical circuit connected to the probe. Since the dielectric properties of the stored material depend on factors other than level, including moisture content and density, this type of device does not provide an absolute measure of level, and requires recalibration when used for material with nonconstant dielectric properties.
Ultrasonic echo detectors utilize the pulse echo technique to determine the distance from the ultrasonic transducer to the surface of the material whose level is to be measured. This device suffers severe limitations when used with granular materials in a dusty environment, due to high scattering loss at the surface of the material and high attenuation of the ultrasonic signal by dust-laden air.
Pressure transducers placed at the bottom of the storage silo produce an electrical signal proportional to the pressure in the material at that point. For most granular and powdered materials the level of the material cannot be deduced from this pressure, since pressure is a nonlinear function of depth, due to friction within the material.
Mechanical ranging devices consist of many variations of a weight suspended on a cable which is lowered until contact with the material is detected by loss of tension in the cable or other means. This device cannot provide continuous measurement due to its slow sampling rate and is unsuitable for use in environments containing abrasive dust, because of its susceptibility to mechanical breakdown.
A further approach for measuring the level of material in a storage silo or tank is illustrated in U.S. Pat. No. 3,695,107, Hertz et al, issued Oct. 3, 1972. In this patent, a delay line, consisting of an outer tubular conductor 2 and a central inner conductor 3, is disposed in the storage silo along the complete length thereof. A pulse is transmitted into the delay line from the , top end thereof, and the pulse is reflected from the air material border in the silo. The time taken for the pulse to travel to the air/material border and back is measured to thereby determine the distance from the top of the storage silo to the top of the material.
Impedance 11 is included in the bottom end of the delay line to prohibit reflections from the bottom of the delay line.
U.S. Pat. No. 4,135,397, Krake, issued Jan. 23, 1979, teaches a refinement of the system taught in the '107 patent and also includes an auxiliary transmission line 34 which is used to calibrate the measuring system.
Although the '107 and '397 patents use terminating impedances, the impedances are matched to the characteristic impedance of their respective lines presumably in open air. However, as is well known, the impedance of the line will be altered when the line is disposed in the material of the silo. Accordingly, the terminating impedances in the '107 and '397 patents will not completely eliminate reflections from the bottom ends of their respective lines.
The above described time domain devices are difficult to build because of the need to resolve sub-nanosecond signals for useful level resolution.
In addition, none of the patents above discussed utilize a swept frequency generator to generate a signal swept in frequency along a transmission line disposed in a silo.